1. Technical Field
The present invention relates to a display device and in particular, relates to a terminal electrode thereof for connecting with an external wiring board.
2. Background Art
In recent years, a display device, especially flat panel display device is widely used as a high-resolution display. A liquid crystal display (LCD) device is one of these flat panel display devices. This LCD device has a substrate (hereinafter, referred to as a TFT substrate) in which switching elements such as thin film transistors (TFTs) are formed and an opposed substrate (hereinafter, referred to as a CF substrate) in which a color filter and a black matrix or the like are formed. The LCD device enables a display by changing an electric field applied to liquid crystal sandwiched between the TFT substrate and the CF substrate to change an alignment direction of molecules of the liquid crystal and thereby controlling amount of transmission light for every pixel. The TFT substrate is connected to an external wiring board in which a drive circuit is provided to apply the electric field to the liquid crystal. A TCP (Tape Carrier Package), a COG (Chip On Glass) and a COF (Chip On Film) or the like are employed as the external wiring board.
FIG. 10 shows a cross sectional view of an ordinary LCD device adopting the TCP as the external wiring board. The size of the TFT substrate 1 is made larger than the CF substrate 13 as shown in FIG. 10 to provide a contact area for the TCP. An area where the TFT substrate 1 and the CF substrate 13 oppose via a liquid crystal material 14 is called a display area. The remaining area is called a peripheral area which includes a terminal area of the TFT substrate 1 extended from the CF substrate 13 and an area of a sealant 15 adjacent to this display area 16B. A group of terminal electrodes derived from the display area is formed on the terminal area of the TFT substrate. In the display area 16B, data lines, scanning lines, power source wiring, etc. are formed on the TFT substrate 1 and they are lead out to the terminal area by way of lead area. In FIG. 10, all of the terminal electrodes, lead electrodes in the lead area and those wiring lines associated with TFTs and pixel electrodes in the display area are formed in common by a metal wiring layer 2. The terminal electrodes formed on an end side of this metal wiring layer 2 are electrically connected to a TCP 8 via a TCP wiring terminal 9 thereof by using an anisotropic conducting film (hereafter, referred to as an ACF) 7.
Details around a contact portion between the TFT substrate 1 and the TCP 8 in the terminal area in FIG. 10 are shown in FIG. 11. As shown in FIG. 11A and FIG. 11B, the metal wiring layer 2 is coated with an insulating layer 4 for protecting the metal layer 2. And a contact hole 5 is formed in the insulating layer 4 to expose the metal wiring layer 2 at a portion of each terminal electrode. The contact hole 5 is coated with a surface conductive layer 6 made of indium tin oxide (ITO) or the like to provide connecting regions for electrically connecting the TFT substrate 1 and the TCP 8.
When the TCP 8 is connected to the terminal electrode 2, the contact hole 5 is covered with the surface conductive layer 6 and the ACF 7. However, the moisture permeation depression effect of the ITO is low, and the ACF 7 has a certain amount of water absorption. Actually, as shown in FIG. 12A and FIG. 12B, the misplacement tends to occur in a case where the metal wiring layer 2 and the TCP wiring 9 are press-welded due to variation in alignment accuracy of a pressure welding equipment used in the TCP pressure welding. The corrosion due to the misplacement mentioned above tends to occur at an edge portion of the contact hole 5 which is usually not covered with the insulating layer 4. Therefore, it becomes important to prevent disconnection of the terminal electrode 2 due to corrosion, and to suppress the display failure.
In an LCD device, a wiring metal of low electric resistivity is adopted for high definition display with a large screen. Therefore, instead of corrosive resistant wiring materials, such as Cr (chromium), low electric resistive metal such as Al (aluminum) and Cu (copper) are used despite of low corrosion resistance. Accordingly, corrosion prevention measures for the terminal electrode 2 have become increasingly important under such circumstances.
An example of a related technology of such measure is disclosed in Japanese Patent Application Laid-Open No 2004-205550. A terminal electrode of an image display device described in this gazette has a configuration that an opening (a contact hole) not covered with an insulating layer on a conductive layer is arranged so that it may bring near by one edge of the conductive layer. It is supposed that the time required for a corrosion reaction occurred at a portion of the opening to reach both ends of the conductive layer may be delayed, and thus a disconnection of the terminal electrode can be suppressed by this configuration.